Renal transplantation represents first-line therapy for patients with ESRD, with the most recent data documenting significant one-year success rates (with 94% graft survival for deceased donor transplants, and 98% graft survival with living donor allografts). However, patients continue to face high morbidity and mortality after transplant, both from chronic allograft rejection and from the toxicities associated with standard immunosuppressive regimens. Given these dual risks, the ultimate goal in this field is the induction of immune tolerance after transplantation, which promises life-long acceptance of an allograft, without the need for ongoing immunosuppression and importantly, with preservation of protective immunity. While novel pharmacologic and antibody-based therapies represent real promise in terms of greatly prolonging allograft survival and reducing off-target toxicities, murine studies strongly suggest that the most robust strategies for inducing immune tolerance incorporate cellular therapies, including both mixed chimerism induction (especially in combination with T cell costimulation blockade) and reaulatory T cell (Treq)-based approaches. Both of these therapies hold the promise of fundamentally resetting recipient immunity in favor of allograft acceptance and thus provide a direct pathway towards immune tolerance induction. Therefore, in this application, we will address mixed-chimerism- and Treg-based tolerance induction through the following Specific Aims: Specific Aim #1: To induce immune tolerance to a renal allograft through the induction of stable, full spectrum mixed-hematopoietic chimerism in the setting of T cell costimulation blockade. Specific Aim #2: To utilize regulatory T cells to consolidate mixed-chimerism and to induce immune tolerance during renal transplantation.